The present invention concerns a method for determining the optimal length of a receive data block in a time-division multiple access (TDMA) communication system.
In a time-division multiple access (TDMA) communication system the structure of the signal is organized into time slots and the time spreads in multipath channels cause intersymbol interference if they exceed the symbol interval, in which case an equalizer must be used.
To each time slot there corresponds a data block comprising information data symbols and a training sequence known to the receiver. The training sequence is usually placed in the middle of the data block in order to minimize the effect of channel fluctuations that the receiver must allow for. A data block usually comprises start symbols, stop symbols, data symbols and a training sequence inserted in the middle of the data. The size of a data block is a compromise between:
the requirements of the service, namely the number of data symbols to be transmitted per block; PA1 the rate of variation of the communication channel; and PA1 the efficiency required of the radio channel, which is defined by the maximal value of the ratio of the number of data items to the size of the block.
An equalizer is used to correct intersymbol interference in a receiver. To operate correctly it must know the impulse response of the transmission channel. To this end, special symbols known to the receiver are transmitted in the training sequence. It is assumed that the symbols of the data transmitted are not known to the receiver.
A training sequence is usually made up of K precursor symbols, P reference symbols and optionally K "post-cursor" symbols, where K is the length of the transmission channel which is defined as the number of symbol intervals equivalent to the difference between the longest path and the shortest path of the channel, with P.gtoreq.K+1.
The training sequence is chosen to suit the transmission channel and in particular its length.
A channel estimator device is used in the receiver to establish the impulse response of the channel. It generates a replica of the training sequence and correlates it with the respective received symbol sequence. The result of this correlation is a set of coefficients h.sub.i with i varying from 0 through L where L is the channel length. This set of coefficients is then supplied to the equalizer. The most direct path on the channel is represented by ho, whereas the other coefficients represent longer paths which interfere with the first.
It is essential to size the data sequences of a block in such a way as to reconcile a sufficient or specified efficiency of the radio channel and a valid estimated impulse response of the channel for all data symbols including those at both ends of the block and therefore farthest away from the training sequence.
A first method of determining the length of a data block is to use simulation results, such as those described in the article "An investigation of block-adaptive decision feedback equalization for frequency selective fading channels" by G. W. DAVIDSON, D. D. FALCONER and A. U. H. SHEIK, Canadian J. Elect. & Comp. Eng, Vol.13 No.3-4, 1988, which discloses simulation results for a decision feedback equalizer emphasizing the influence of the training sequence length on receiver performance.
A second method of determining an optimal length is to base this determination on the results of measurements carried out in the field. This method has the major drawback of being costly and of being dependent on specific environment conditions.